1. Field of the Invention
This invention relates generally to the fields of neurobiology and medicine and, more specifically, to the field of ionotropic receptors.
2. Background Information
Ionotropic glutamate receptors activate ligand-gated cation channels that mediate the predominant component of excitatory neurotransmission in the central nervous system (CNS). These receptors have been classified based on their preference for the glutamate-like agonists (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA), kainate (KA), and N-methyl-D-aspartate (NMDA). All three glutamate receptor subtypes are heteromultimeric complexes, and many of the subunits that comprise them have been identified and characterized. To date, four AMPA receptor subunits (GluR1-4), five KA receptor subunits (GluR5-7, KA1, and KA2), and six NMDA receptor subunits (NR1, NR2A-2D and NR3A) have been reported.
The NMDA receptor (NMDAR) has unique properties that distinguish it from the other glutamate receptor subtypes. First, the activation of NMDAR requires the presence of dual agonists, glutamate (or NMDA) and glycine. In addition, activation of these receptors is regulated by Mg2+ in a voltage-dependent manner (i.e., the NMDAR is blocked at resting membrane potential and activated when depolarized). Most importantly, the NMDAR is extremely permeable to Ca2+, a key regulator of cell function. These unique properties allow NMDARs to play a critical role in development of the nervous system, synaptic plasticity, memory, and other physiological processes in the CNS. However, excessive stimulation of NMDARs has also been implicated in many pathological conditions including stroke, ischemia, head and spinal trauma, headache, epilepsy, neuropathic pain syndromes including diabetic neuropathy, glaucoma, depression and anxiety, drug addiction/withdrawal/tolerance, and in chronic neurodegenerative states, such as Alzheimer's disease, Huntington's disease, HIV-associated dementia, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis (ALS).
The molecular cloning and functional analysis of expressed NR1, NR2A-D, and NR3A subunits, coupled with the examination of their temporal and spatial expression patterns in vivo, has led to significant advances in our understanding of NMDAR function at the molecular level. However, the identification of these six subunits alone has failed to explain the observed diversity in NMDAR function, particularly in motor neurons.
Thus, there exists a need to identify and characterize additional NMDAR subunits, and to characterize the function of additional NMDA receptors. There also exists a need to provide screening assays that identify compounds that modulate the function of additional NMDA receptors. Such compounds can be used to treat pathological conditions in which inappropriate NMDA receptor activation, or inappropriate responses to glycine or glutamate, are involved. The present invention satisfies these needs and provides related advantages as well.